As design rules decrease, it becomes more difficult to form stripes of metallization. Also, as device sizes become smaller, the resistance of each conductive interconnect itself can no longer be neglected. Therefore, it has been required that each conductive interconnect be made of a material having a minimum resistance available. Examples of the metallization material having small resistance include aluminum and materials consisting mainly of aluminum.
However, where stripes of metallization are made of a metallic material consisting principally of aluminum, the aluminum component grows abnormally, thus deforming the stripes of metallization with undesired convex surface protrusions. These are usually called hillocks and whiskers. Furthermore, undesired shapes are created.
These hillocks and whiskers are produced during heating for formation of films, during heating for ashing of resist (removal of resist by an oxygen plasma), and during heating induced by laser irradiation used for annealing.
The hillocks are produced by abnormal growth of aluminum. In particular, when aluminum components grow abnormally locally, the locally growing portions collide against each other, thus resulting in convex protrusions. The whiskers are needle-like or rectangular protrusions produced by abnormal growth of aluminum. The causes of the hillocks and whiskers are not understood exactly, but it is considered that some impurities in the aluminum or nonuniformity of the crystalline structure of the aluminum induces them.
These hillocks and whiskers grow over lengths of several micrometers and, therefore, where an integrated circuit comprising a number of conductive interconnects and components spaced only several micrometers from each other is fabricated, the hillocks and whiskers are great impediments.
One method of suppressing the hillocks and whiskers is to add a trace amount of a rare-earth element, silicon, or other element to the aluminum. However, where the element is heated to a temperature exceeding about 400.degree. C., hillocks and whiskers are again produced.
Moreover, there is an increasing demand for formation of aluminum interconnects such as gate interconnects in the early stage of the fabrication process. In this case, the problems of the hillocks and whiskers cause more serious results, because the aluminum interconnects are unavoidably frequently subjected to heating during heating steps of the process or during other steps inevitably involving heating such as ion implantation.
The hillocks and whiskers are problematic because the distance between vertically or horizontally spaced conductive interconnects might be shorted by them. As design rules and interconnect pitch diminish, this problem becomes more conspicuous. Especially, where the interconnect pitch is reduced below 2 .mu.m, shorting of adjacent conductive interconnects and shorting of vertically adjacent conductive interconnects due to lateral hillocks and whiskers pose problems.
In locations where conductive interconnects existing in different layers intersect each other, it is necessary to form an interlayer dielectric film (made of a silicon oxide film, for example) on the lower layer of metallization and to form the upper layer of metallization on the interlayer dielectric film. In this case, if the step coverage of the interlayer dielectric film is not good, then the upper layer of metallization will break at the step or local resistance increases will be induced. If a metallization layer made of aluminum or a material consisting mainly of aluminum is formed, followed by formation of an interlayer dielectric film, and if the second layer of metallization is subsequently formed, then the step coverage of the interlayer dielectric film is deteriorated by the hillocks and whiskers inevitably produced as mentioned above. As a result, the second layer of metallization formed on the interlayer dielectric film breaks at the step or present other problems.
Another technique for solving this problem has been proposed. Specifically, conductive interconnects are formed from a metallic material capable of being anodized such as aluminum. Using the interconnects as anodes, an anodization process is carried out. An anodic oxide film is formed on the exposed surfaces of the conductive interconnects. For example, where the conductive interconnects are made of aluminum or a material consisting principally of aluminum, an oxide film made of aluminum or a material consisting mainly of aluminum is formed on the top and side surfaces of the interconnects. This strengthens the top and side surfaces of the interconnects, thus suppressing the generation of the hillocks and whiskers.
However, in order to effect the anodization, a pattern different from the actual circuit interconnection pattern must be formed. After the anodization, the required conductive pattern must be created by etching techniques to make it possible to supply electrical current to every conductive interconnect. This means an increase in the number of manufacturing steps and hence not desirable. Especially, the former patterning step is performed after formation of the circuit conductive interconnects and so unwanted etching tends to occur. Consequently, this is not desirable from a view point of manufacturing yield.
In addition, as design rules and linewidths decrease, stress induced during anodization often causes conductive interconnects to be deformed and break. The problem especially becomes more conspicuous where the geometry of the conductive interconnects becomes more complex.
Further, as design rules and linewidths decrease, a voltage drop caused during anodization by the conductive interconnect resistance produces an effect. That is, as the voltage drops, the thickness of the formed anodic oxide film is varied.
This problem may be solved by increasing the cross-sectional area of each conductive interconnect to more than needed so as to alleviate the voltage drop during the anodization due to the conductive interconnect resistance. However, increasing the cross-sectional area of the interconnects hinders increasing the circuit integration density.
Anodic oxidation techniques can prevent hillocks and whiskers where conductive interconnects or electrodes are formed from aluminum or a material consisting mainly of aluminum. However, the aforementioned various problems take place. Besides aluminum, conductive materials capable of being anodized such as tantalum are known. The above-described problems again occur where these materials are used.